238 lines
7.7 KiB
Plaintext
238 lines
7.7 KiB
Plaintext
unit imjfdctfst;
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{ This file contains a fast, not so accurate integer implementation of the
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forward DCT (Discrete Cosine Transform).
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A 2-D DCT can be done by 1-D DCT on each row followed by 1-D DCT
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on each column. Direct algorithms are also available, but they are
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much more complex and seem not to be any faster when reduced to code.
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This implementation is based on Arai, Agui, and Nakajima's algorithm for
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scaled DCT. Their original paper (Trans. IEICE E-71(11):1095) is in
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Japanese, but the algorithm is described in the Pennebaker & Mitchell
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JPEG textbook (see REFERENCES section in file README). The following code
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is based directly on figure 4-8 in P&M.
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While an 8-point DCT cannot be done in less than 11 multiplies, it is
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possible to arrange the computation so that many of the multiplies are
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simple scalings of the final outputs. These multiplies can then be
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folded into the multiplications or divisions by the JPEG quantization
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table entries. The AA&N method leaves only 5 multiplies and 29 adds
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to be done in the DCT itself.
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The primary disadvantage of this method is that with fixed-point math,
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accuracy is lost due to imprecise representation of the scaled
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quantization values. The smaller the quantization table entry, the less
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precise the scaled value, so this implementation does worse with high-
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quality-setting files than with low-quality ones. }
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{ Original: jfdctfst.c ; Copyright (C) 1994-1996, Thomas G. Lane. }
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interface
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{$I imjconfig.inc}
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uses
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imjmorecfg,
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imjinclude,
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imjpeglib,
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imjdct; { Private declarations for DCT subsystem }
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{ Perform the forward DCT on one block of samples. }
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{GLOBAL}
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procedure jpeg_fdct_ifast (var data : array of DCTELEM);
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implementation
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{ This module is specialized to the case DCTSIZE = 8. }
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{$ifndef DCTSIZE_IS_8}
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Sorry, this code only copes with 8x8 DCTs. { deliberate syntax err }
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{$endif}
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{ Scaling decisions are generally the same as in the LL&M algorithm;
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see jfdctint.c for more details. However, we choose to descale
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(right shift) multiplication products as soon as they are formed,
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rather than carrying additional fractional bits into subsequent additions.
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This compromises accuracy slightly, but it lets us save a few shifts.
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More importantly, 16-bit arithmetic is then adequate (for 8-bit samples)
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everywhere except in the multiplications proper; this saves a good deal
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of work on 16-bit-int machines.
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Again to save a few shifts, the intermediate results between pass 1 and
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pass 2 are not upscaled, but are represented only to integral precision.
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A final compromise is to represent the multiplicative constants to only
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8 fractional bits, rather than 13. This saves some shifting work on some
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machines, and may also reduce the cost of multiplication (since there
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are fewer one-bits in the constants). }
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const
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CONST_BITS = 8;
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const
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CONST_SCALE = (INT32(1) shl CONST_BITS);
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const
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FIX_0_382683433 = INT32(Round(CONST_SCALE * 0.382683433)); {98}
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FIX_0_541196100 = INT32(Round(CONST_SCALE * 0.541196100)); {139}
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FIX_0_707106781 = INT32(Round(CONST_SCALE * 0.707106781)); {181}
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FIX_1_306562965 = INT32(Round(CONST_SCALE * 1.306562965)); {334}
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{ Descale and correctly round an INT32 value that's scaled by N bits.
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We assume RIGHT_SHIFT rounds towards minus infinity, so adding
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the fudge factor is correct for either sign of X. }
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function DESCALE(x : INT32; n : int) : INT32;
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var
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shift_temp : INT32;
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begin
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{ We can gain a little more speed, with a further compromise in accuracy,
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by omitting the addition in a descaling shift. This yields an incorrectly
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rounded result half the time... }
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{$ifndef USE_ACCURATE_ROUNDING}
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shift_temp := x;
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{$else}
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shift_temp := x + (INT32(1) shl (n-1));
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{$endif}
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{$ifdef RIGHT_SHIFT_IS_UNSIGNED}
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if shift_temp < 0 then
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Descale := (shift_temp shr n) or ((not INT32(0)) shl (32-n))
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else
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{$endif}
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Descale := (shift_temp shr n);
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end;
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{ Multiply a DCTELEM variable by an INT32 constant, and immediately
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descale to yield a DCTELEM result. }
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function MULTIPLY(X : DCTELEM; Y: INT32): DCTELEM;
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begin
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Multiply := DeScale((X) * (Y), CONST_BITS);
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end;
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{ Perform the forward DCT on one block of samples. }
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{GLOBAL}
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procedure jpeg_fdct_ifast (var data : array of DCTELEM);
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type
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PWorkspace = ^TWorkspace;
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TWorkspace = array [0..DCTSIZE2-1] of DCTELEM;
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var
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tmp0, tmp1, tmp2, tmp3, tmp4, tmp5, tmp6, tmp7 : DCTELEM;
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tmp10, tmp11, tmp12, tmp13 : DCTELEM;
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z1, z2, z3, z4, z5, z11, z13 : DCTELEM;
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dataptr : PWorkspace;
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ctr : int;
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{SHIFT_TEMPS}
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begin
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{ Pass 1: process rows. }
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dataptr := PWorkspace(@data);
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for ctr := DCTSIZE-1 downto 0 do
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begin
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tmp0 := dataptr^[0] + dataptr^[7];
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tmp7 := dataptr^[0] - dataptr^[7];
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tmp1 := dataptr^[1] + dataptr^[6];
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tmp6 := dataptr^[1] - dataptr^[6];
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tmp2 := dataptr^[2] + dataptr^[5];
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tmp5 := dataptr^[2] - dataptr^[5];
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tmp3 := dataptr^[3] + dataptr^[4];
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tmp4 := dataptr^[3] - dataptr^[4];
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{ Even part }
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tmp10 := tmp0 + tmp3; { phase 2 }
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tmp13 := tmp0 - tmp3;
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tmp11 := tmp1 + tmp2;
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tmp12 := tmp1 - tmp2;
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dataptr^[0] := tmp10 + tmp11; { phase 3 }
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dataptr^[4] := tmp10 - tmp11;
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z1 := MULTIPLY(tmp12 + tmp13, FIX_0_707106781); { c4 }
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dataptr^[2] := tmp13 + z1; { phase 5 }
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dataptr^[6] := tmp13 - z1;
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{ Odd part }
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tmp10 := tmp4 + tmp5; { phase 2 }
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tmp11 := tmp5 + tmp6;
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tmp12 := tmp6 + tmp7;
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{ The rotator is modified from fig 4-8 to avoid extra negations. }
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z5 := MULTIPLY(tmp10 - tmp12, FIX_0_382683433); { c6 }
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z2 := MULTIPLY(tmp10, FIX_0_541196100) + z5; { c2-c6 }
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z4 := MULTIPLY(tmp12, FIX_1_306562965) + z5; { c2+c6 }
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z3 := MULTIPLY(tmp11, FIX_0_707106781); { c4 }
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z11 := tmp7 + z3; { phase 5 }
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z13 := tmp7 - z3;
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dataptr^[5] := z13 + z2; { phase 6 }
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dataptr^[3] := z13 - z2;
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dataptr^[1] := z11 + z4;
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dataptr^[7] := z11 - z4;
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Inc(DCTELEMPTR(dataptr), DCTSIZE); { advance pointer to next row }
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end;
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{ Pass 2: process columns. }
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dataptr := PWorkspace(@data);
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for ctr := DCTSIZE-1 downto 0 do
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begin
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tmp0 := dataptr^[DCTSIZE*0] + dataptr^[DCTSIZE*7];
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tmp7 := dataptr^[DCTSIZE*0] - dataptr^[DCTSIZE*7];
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tmp1 := dataptr^[DCTSIZE*1] + dataptr^[DCTSIZE*6];
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tmp6 := dataptr^[DCTSIZE*1] - dataptr^[DCTSIZE*6];
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tmp2 := dataptr^[DCTSIZE*2] + dataptr^[DCTSIZE*5];
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tmp5 := dataptr^[DCTSIZE*2] - dataptr^[DCTSIZE*5];
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tmp3 := dataptr^[DCTSIZE*3] + dataptr^[DCTSIZE*4];
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tmp4 := dataptr^[DCTSIZE*3] - dataptr^[DCTSIZE*4];
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{ Even part }
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tmp10 := tmp0 + tmp3; { phase 2 }
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tmp13 := tmp0 - tmp3;
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tmp11 := tmp1 + tmp2;
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tmp12 := tmp1 - tmp2;
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dataptr^[DCTSIZE*0] := tmp10 + tmp11; { phase 3 }
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dataptr^[DCTSIZE*4] := tmp10 - tmp11;
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z1 := MULTIPLY(tmp12 + tmp13, FIX_0_707106781); { c4 }
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dataptr^[DCTSIZE*2] := tmp13 + z1; { phase 5 }
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dataptr^[DCTSIZE*6] := tmp13 - z1;
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{ Odd part }
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tmp10 := tmp4 + tmp5; { phase 2 }
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tmp11 := tmp5 + tmp6;
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tmp12 := tmp6 + tmp7;
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{ The rotator is modified from fig 4-8 to avoid extra negations. }
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z5 := MULTIPLY(tmp10 - tmp12, FIX_0_382683433); { c6 }
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z2 := MULTIPLY(tmp10, FIX_0_541196100) + z5; { c2-c6 }
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z4 := MULTIPLY(tmp12, FIX_1_306562965) + z5; { c2+c6 }
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z3 := MULTIPLY(tmp11, FIX_0_707106781); { c4 }
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z11 := tmp7 + z3; { phase 5 }
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z13 := tmp7 - z3;
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dataptr^[DCTSIZE*5] := z13 + z2; { phase 6 }
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dataptr^[DCTSIZE*3] := z13 - z2;
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dataptr^[DCTSIZE*1] := z11 + z4;
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dataptr^[DCTSIZE*7] := z11 - z4;
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Inc(DCTELEMPTR(dataptr)); { advance pointer to next column }
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end;
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end;
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end.
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